Recovery mechanisms in multi-layer communication networks are among the features most frequently requested by network operators for OPEX and CAPEX reduction in transport networks. The most classic example is packet-optical integration, where the packet layer control plane needs fully diverse paths in the optical layer; so that a single fault in the optical layer does not affect multiple branches of a protection scheme in the packet layer.
The Internet Engineering Task Force, IETF, has been working on this for long time and two solutions have been standardized for path diversity in distributed control plane environments. In the first, GMPLS UNI, defined in RFC4203, RSVP-TE extensions have been defined for the collection of Shared Risk Link Groups, SRLGs. The procedure envisages the packet domain requesting setup of a label switched path, LSP, in the optical domain, collecting the SRLGs of a given LSP and performing a second (or further) request asking to avoid SRLGs during the computation of the path. In the second, PCEP, defined in RFC4927, allows a single request to be made to the optical layer specifying the need for two or more diverse paths. The identifiers of such LSPs are returned to the packet domain, which does not have any kind of information about the paths except that they are diverse.
The market trends are moving from the fully distributed control planes of Internet Protocol/Multiprotocol label switching, IP/MPLS, and generalized MPLS, GMPLS, to transport software-defined networking, SDN. In SDN, multi-layer networks can be managed by a single SDN controller or by a hierarchy of SDN controllers.
The IETF is currently working on a framework standardizing an interface between SDN controllers; this is called the Abstraction and Control of Transport Networks, ACTN, framework and is described in draft IETF document draft-ceccarelli-teas-actn-framework-00.txt of 15 Jun. 2015.
The ACTN framework defines two types of SDN controller: the Physical Network Controller, PNC; and the Multi Domain Service Controller (or Coordinator), MDSC. The PNC is often provided by the same vendor of the network to be controlled, mostly in the optical case, and often proprietary interfaces are used between the controller and the nodes. By means of a standard interface, which does not deal with physical impairments of nodes specific characteristics and based on standard models, it is possible to have a parent SDN controller, the MDSC, which allows for end to end path computation and provisioning based on an abstracted view of the network.
The current ACTN definitions work well for multi-domain environments, where the MDSC provides a summarized view of a multi-domain networks and the PNCs are allowed to ignore information about each other, because the sub-networks are on the same layer and independent. More complete support for multi-layer arrangements is desirable.